In the investment casting of molten metal or alloy (melt), a ceramic gang mold has been employed where the gang mold comprises a melt pour cup connected to a plurality of individual article-forming molds. In one conventional casting set-up for casting gas turbine engine blades, the pour cup includes multiple main melt supply gating spokes extending therefrom. The main gating spokes each in turn branch into multiple individual melt supply gating spokes each of which extends to a respective individual mold. For example, three or more melt supply branch gating spokes may branch from each main spoke. Each article-forming mold includes at least one mold cavity having the shape of the article to be cast.
In a conventional casting set-up, the initial stream of melt poured from a crucible into the pour cup of the mold is usually narrow to insure that the melt is received in the pour cup and that there is minimum splashing of the melt inside the casting furnace. The main gating spokes are communicated to the pour cup and typically serve the dual purpose of delivery of melt to a respective individual mold via a respective branch gating spoke and of providing an adequate reservoir of melt to compensate for the liquid-to-solid shrinkage in the mold. For example, in casting equiaxed grain articles, the cross-sectional area of the spokes therefore typically has to be larger than the cross-section of the mold cavity that is filled from the spoke. For example, in a conventional equiaxed casting set-up, the collective cross-sections of the spokes can be at least ten times larger than the initial pour stream. Such a large ratio of spoke-to-initial pour stream cross-sectional areas results in inconsistent and uneven delivery of melt among the molds. The spokes that are oriented in the direction of the pour stream receive more metal flow than those spokes which are located away from the direction of pour stream, resulting in uneven initial fill of the molds.
In the above conventional casting set-up, the need for a large number of spokes to provide both initial melt delivery and melt feeding to accommodate solidification shrinkage of individual molds is disadvantageous from the standpoint that metal or alloy usage is inefficient. That is, the metal or alloy solidified in the main and branch spokes is not cast into a usable article but remains as individual gating of the molds.
The gang mold has been formed by the well known lost wax process where a wax or other fugitive pattern assembly corresponding to the gang mold features is repeatedly dipped in ceramic slurry, drained of excess slurry, and stuccoed with coarse ceramic stucco particulates to build up a desired thickness of ceramic shell thickness on the pattern assembly. The pattern assembly then is selectively removed, and the remaining ceramic gang shell mold is heated at elevated temperature to impart strength properties to the shell mold needed for subsequent casting. During casting, molten metal or alloy is poured into the pour cup and flows via the gating to fill the article-forming molds substantially concurrently. The molten metal or alloy is solidified in the molds to form an investment cast article in the molds.
In the investment casting of critical aerospace components, such as gas turbine engine blades, vanes and the like, the gang molds oftentimes include a molten metal or alloy filter placed in each melt feed runner supplying molten metal or alloy from the pour cup to the spokes in order to remove non-metallic inclusions from the molten metal or alloy before it enters the individual molds.